It's 15% of the true damage that this item deals, so 15% of 40 - 140 would be 6 - 26 damage. This isn't an ideal item to use against Grumpjaw because Grumpjaw is already very bulky (perk aside), and true damage deal damage directly ignoring shield and armor. Basically the bulkier the enemy the less effective this item gets. The best thing to against Grumpjaw would be any type of armor pierce/shred. Good items would be Bonesaw, Poisoned Shiv, and Tension Bow.

This item is typically used for Roam type characters. Ever played Phinn or Ardan or (pick one of the support characters) and tried to take down a jungle camp by yourself with a typical roam build? Ever gone to cover the lane for your laner while they shopped or respawned? It's nauseating how long it takes to take down a minion without the help of your Carry. This item changes that. It's stats are also beneficial with the 3rd highest cooldown percentage, and in a tie for 2nd for energy recharge. Only Clockwork has a higher recharge. So You could build Clockwork for 300 more gold, and keep thumping away at camps and objectives with your styrofoam fists, or you could build this and give yourself a little oomph when taking down a turret.

If you do 100 damage per attack, but only attack once per second, you will do 100 dps (damage per second) but if you have enough attack speed to attack twice per second, you will do 200 dps but for this item, it's capped at 160 dps.

Nope, it's DPS, so it doesn't matter how fast you attack. Like if you attack once a second, it does 150 damage for each attack. If you attack twice a second, it does 75 damage for each attack. If it was like what you said, then everyone would build a stormcrown and tornado triggers for maximum damage.

It gives 1100 gold of energy/cooldown stats, 1400 gold of damage, and 225 gold of health, for a total of 2725 gold worth of stats. Basically, get it when you want aftershock but don't have good ratios/pierce/crits as jungle/roam.

It also gives 4x dmg vs minions/turrets, of course. But that's unimportant imo, because clear speed is fast enough for you to farm all the jungle monsters before they respawn anyhow.